Rankine cycle apparatus have been known as systems for converting heat energy into mechanical work. The Rankine cycle apparatus include a structure for circulating water in the liquid and gaseous phases within a sealed piping system forming a circulation system in the apparatus. Generally, the Rankine cycle apparatus include a water supplying pump unit, an evaporator, an expander, a condenser, and pipes connecting between these components to provide circulation circuitry.
FIG. 13 is a schematic block diagram of a general setup of a conventionally-known Rankine cycle apparatus (e.g., vehicle-mounted Rankine cycle apparatus) and certain details of a condenser employed in the Rankine cycle apparatus. The Rankine cycle apparatus of FIG. 13 includes a water supplying pump unit 110, an evaporator 111, an expander 107, and the condenser 100. These components 110, 111, 107 and 100 are connected via pipes 108 and 115, to provide circulation circuitry in the apparatus.
Water (liquid-phase working medium), which is supplied, a predetermined amount per minute, by the water supplying pump unit 110 via the pipe 115, is imparted with heat by the evaporator 111 to turn into water vapor (gaseous-phase working medium). The vapor is delivered through the next pipe 115 to the expander 107 that expands the water vapor. Mechanical device (not shown) is driven through the vapor expansion by the expander 107 so as to perform desired mechanical work.
Then, the expansion of the water vapor is terminated by lowering the temperature and pressure of the vapor and the resultant water vapor of the lowered temperature and pressure is delivered through the pipe 108 to the condenser 100, where the vapor is converted from the vapor phase back to the water phase. After that, the water is returned through the pipe 115 to the water supplying pump unit 110, from which the water is supplied again for repetition of the above actions. The evaporator 111 is constructed to receive heat from an exhaust pipe extending from the exhaust port of the engine of the vehicle.
Among various patent documents showing examples of the Rankine cycle apparatus is Japanese Patent Laid-Open Publication No. 2002-115504.
The following paragraphs detail a structure and behavior of the condenser 100 in the conventional vehicle-mounted Rankine cycle apparatus shown in FIG. 13.
The condenser 100 includes a vapor introducing chamber 101, a water collecting chamber 102, and a multiplicity of cooling pipes 103 vertically interconnecting the two chambers 101 and 102. In the figure, only one of the cooling pipes 103 is shown in an exaggerative manner. Substantial upper half of the interior of each of the cooling pipes 103 is a vapor (gaseous-phase) portion 104, while a substantial lower half of the interior of the cooling pipe 103 is a water (liquid-phase) portion 105. In the vapor portion 104, most of the working media introduced via the vapor introducing chamber 101 to the cooling pipe 103 is in the gaseous phase, while, in the water portion 105, most of the working media flowing through the cooling pipe 103 is kept in the liquid (condensed water) phase. Boundary between the vapor 104 and the water 105 is a liquid level position 112.
One cooling fan 106 is disposed behind the cooling pipes 103 (to the right of the cooling pipes 103 in FIG. 13). Normally, operation of the cooling fan 106 is controlled by an electronic control unit on the basis of a water temperature at an outlet port of the condenser 100. The single cooling fan 106 sends air to the entire region, from top to bottom, of all of the cooling pipes 103 to simultaneously cool the cooling pipes 103.
The condenser 100 operates as follows during operation of the Rankine cycle apparatus. Water vapor of a relatively low temperature, discharged from the expander 107 with a reduced temperature and pressure, is sent into the vapor introducing chamber 101 of the condenser 100 via the low-pressure vapor pipe 108 and then directed into the cooling pipes 103. Cooling air 109 drawn into the cooling fan 106 is sent to the condenser 100.
Strong cooling air is applied by the cooling fan 106 to the upstream vapor portion 104 of the condenser 100, i.e. a portion of each of the cooling pipes 103 where a mixture of the vapor and water exists, and thus latent heat emitted when the vapor liquefies can be recovered effectively by the cooling air. Cooling air is also applied by the cooling fan 106 to the downstream water portion 105 of the condenser 100, i.e. a portion of each of the cooling pipes 103 where substantially only the water exists. Water condensed within the cooling pipes 103 of the condenser 100, is collected into the water collecting chamber 102 and then supplied by the water supplying pump unit 110 to the evaporator 111 in a pressurized condition as noted above.
The following paragraphs set forth problems with the above-discussed conventional Rankine cycle apparatus with reference to FIG. 14, where section (A) shows variation in vehicle velocity, section (B) shows variation in engine output, section (C) shows variation in amount of water supply to the evaporator 111 and section (D) shows variation in liquid level position within the condenser 100.
As the vehicle, having the Rankine cycle apparatus mounted thereon, varies its traveling velocity as indicated in section (A) of FIG. 14, the engine output varies as indicated in section (B), in response to which the amount of water supply to the evaporator 111 varies as indicated in section (C) and the liquid level position within the condenser 100 varies in section (D). More specifically, as the vehicle starts traveling at time points t1, t3 and t5 and stops traveling at time points t2, t4 and t6 along the horizontal time axis, the engine output and the amount of water supply to the evaporator 111 vary, in response to which the liquid level position 112 within the condenser 100 varies.
Namely, because the engine output varies as indicated in section (B) in response to transitional vehicle velocity variation at the time of start, stop, etc. of the vehicle as indicated in section (A), the condenser 100 of the conventional vehicle-mounted Rankine cycle apparatus causes the amount of water supply to the evaporator 111 to vary, which results in variation in the liquid level position 112 within each of the cooling pipes 103 of the condenser 100. Namely, in the condenser 100, the liquid level position 112 rises when the amount of the vapor flowing into the condenser 100 (i.e., inflow amount of the vapor) is greater than the amount of the condensed water discharged from the condenser 100 (i.e., discharge amount of the condensed water), but lowers when the inflow amount of the vapor is smaller than the discharge amount of the condensed water. In this way, the vapor-occupied portion (104) in the cooling pipes 103 of the condenser 100 increases or decreases. Because the condensed water (in the portion 105) is discharged by the water supplying pump unit 110 subjected to predetermined flow rate control, a pressure from an outlet port 113 of the expander 107 to an inlet port 114 of the water supplying pump unit 110 is determined by a pressure within the condenser 100.
The pressure within the condenser 100 is determined by an amount of condensing heat exchange caused by cooling of the vapor portion (104) of the condenser, and the amount of condensing heat exchange is determined by a flow rate of the media to be cooled and a surface area of a condensing heat transmission portion. Thus, if the portion occupied with the vapor increases or decreases due to variation (rise or fall) of the liquid level position 112, the surface area 116 of the condensing heat transmission portion increases or decreases and so the pressure within the condenser 100 and the flow rate of the media to be cooled do not uniformly correspond to each other any longer, as a result of which control of the pressure within the condenser 100 based on adjustment of the flow rate of the media to be cooled would become impossible. In the case where the pressure within the condenser 100 can not be controlled as noted above, an increase in the pressure within the condenser 100 would lead to a decrease in the output of the expander. Further, decrease in the pressure within the condenser 100 produces cavitations (bubbles) in the water supplying pump unit 110, which would result in functional deterioration of the pump unit 110 and adversely influence the durability of the pump unit 110.
Similarly, the temperature of the condensed water at the outlet port of the condenser 100 is determined by an amount of heat exchange caused by cooling of the water portion (105) of the condenser, and the amount of the heat exchange of the condensed water is determined by the flow rate of the media to be cooled and a surface area 117 of a heat transmission portion of the condensed water. Thus, if the portion occupied with the condensed water (105) increases or decreases due to variation (rise or fall) of the liquid level position 112, the surface area 117 of the heat transmission of the condensed water portion increases or decreases and so the temperature of the condensed water and the flow rate of the media to be cooled do not uniformly correspond to each other any longer, as a result of which control of the temperature of the condensed water based on adjustment of the flow rate of the media to be cooled would become very difficult. Increase in the temperature of the condensed water in the condenser 100 produces cavitations (bubbles) in the water supplying pump unit 110, which would result in functional deterioration of the pump unit 110 and adversely influence the durability of the pump unit 110. Conversely, a decrease in the temperature of the condensed water would present the problem that extra heat energy has to be consumed when that water is supplied to and re-heated by the evaporator 11.
Japanese Patent Laid-Open Publication Nos. SHO-63-201492 and HEI-10-185458 disclose techniques directed to avoiding the above-discussed problems, in accordance with which the liquid level position within the cooling pipes 103 of the condenser 100 is adjusted via an adjusting valve provided at the outlet port of the condenser 100.
Namely, the SHO-63-201492 publication discloses an air-cooled high-pressure condenser which includes air blowers, a condensed-water outlet adjusting valve, etc., where the pressure within the condenser is controlled with a pressure greater than the atmospheric pressure and the temperature of the condensed water is supercooled at 100° C. or below. Specifically, in the disclosed air-cooled high-pressure condenser, the pressure within the condenser is controlled by adjustment of an amount of cooling air from the air blowers, and the water level within the condenser is controlled by adjustment of the condensed-water outlet adjusting valve.
Further, a high-pressure condenser disclosed in the HEI-10-185458 publication includes a first control that compares a difference between gaseous-phase and liquid-phase pressures within the condenser with a predetermined condenser water level setting and controls a condensed-water outlet adjusting valve so that the pressure difference becomes constant, and a second control that compares a gaseous-phase pressure within the condenser with a predetermined pressure setting and controls a condenser cooling fan in such a manner that the gaseous-phase pressure becomes constant.
However, the techniques disclosed in the SHO-63-201492 and HEI-10-185458 publications, which are designed to only adjust the water at the outlet port, can not re-supply or replenish water when the liquid level has lowered.
Variation in the liquid level position 112 within the condenser would vary the respective heat transmission areas (116 and 117) of the gaseous-phase and liquid-phase portions corresponding to the vapor and condensed water in the condenser, which results in significant variation of the vapor pressure and condensed water temperature within the condenser. As a result, the cooling control would be complicated, making good cooling operation difficult to achieve.
Further, in the conventional Rankine cycle apparatus, an increase in the vapor pressure within the condenser would lead to a decrease in the output of the expander, and a decrease in the vapor pressure and an increase in the condensed water temperature within the condenser would produce cavitations in the water supplying pump unit. Besides, for re-heating in the evaporator due to a decrease in the condensed water temperature, extra energy has to be consumed.
For the reasons set forth above, there has been a great demand for a novel technique which can appropriately control the liquid level position of the condensed water within the condenser so that respective variation of the heat transmission areas of the vapor condensing portion and condensed-water cooling portion can be stabilized with a predetermined range and that the pressure within the condenser and the temperature of the condensed water at the condensed water outlet port can be controlled in a stabilized manner via cooling fans provided in association with the vapor condensing portion and condensed-water cooling portion.